Denitration of High Nitrate Bearing Alkaline Waste Using Two Stage Chemical and Biological Process

Abstract

During the operation of radio-chemical plants, low and intermediate level alkaline waste streams containing nitrates is generated. Some of these waste streams contain large concentrations of nitrates exceeding 100,000 ppm and is of highly alkaline nature. To remediate such wastes, a two stage denitration process was developed—with chemical denitration as the first stage, followed by biological denitrification. The chemical denitration process was developed using catalytic reduction technique for destructing the nitrates and converting them to harmless nitrogen gas using a suitable reductant, in the presence of bimetallic Pd-Cu catalysts. The reductant used was formalin (37–41 % formaldehyde) for the experimental work and different reaction parameters were determined to yield a higher reduction. For studies at laboratory scale, approx. 97 % nitrate reduction of synthetic waste prepared using sodium nitrate was achieved in 5 h and denitration of simulated waste resulted in nitrate reduction of approx. 96 % in 6 h and the final nitrate concentration was near 5000 ppm for both cases. According to Indian standards, the maximum permissible limit of nitrate in water is set at 100 ppm and since the reduced concentration doesn’t match the environmentally safe limits, the biological denitrification process was used for further reduction. Biological process was developed for treating effluent containing approx. 5000 ppm of nitrate, formic acid and unreacted formaldehyde, traces of catalyst and NaOH. The bacteria used were denitrifying bacteria. A continuous anaerobic packed bed reactor was used for denitration process.